Tapered junction shield for self-compensation of asymmetry with increasing aspect ratio for tunneling magneto-resistance (TMR) type read head

ABSTRACT

A junction shield (JS) structure is disclosed for providing longitudinal bias to a free layer (FL) having a width (FLW) and magnetization in a cross-track direction between sidewalls in a sensor. The sensor is formed between bottom and top shields and has sidewalls extending from a front side at an air bearing surface (ABS) to a backside that is a stripe height (SH) from the ABS. The JS structure has a single layer (JS 1 ) adjacent to each sensor sidewall and with a magnetization parallel to that of the FL, and a tapered top surface such that JS 1  has decreasing thickness with increasing height from the ABS. As aspect ratio or AR (SH/FLW) increases above 1, longitudinal bias increases proportionally to slow an increase in asymmetry as AR increases, and without introducing a loss in amplitude for a reader with low AR.

This is a continuation of U.S. patent application Ser. No. 16/430,574filed on Jun. 4, 2019, which is a Divisional application of U.S. Ser.No. 15/686,497, filed on Aug. 25, 2017, now issued as U.S. patent Ser.No. 10/319,398, which is herein incorporated by reference in itsentirety, and assigned to a common assignee.

RELATED PATENT APPLICATION

This application is related to U.S. Pat. No. 9,230,577; which isassigned to a common assignee, and herein incorporated by reference inits entirety.

TECHNICAL FIELD

The present disclosure relates to a longitudinal biasing scheme for asensor structure in a TMR reader wherein a junction shield (JS) formedon either side of a free layer has decreasing down-track thickness withincreasing distance (height) from the air bearing surface (ABS) therebycompensating for greater sensor asymmetry as the aspect ratio (stripeheight to free layer width or FLW) becomes larger in advanced devices,especially where FLW is less than 35 nm.

BACKGROUND

In hard disk drive applications, there is a constant drive to increasethe recording areal density to reduce the cost of information storage.The increase in recording areal density is accomplished by decreasingthe size of the writer and reader sensors that are used to record andreproduce signals. In today's products, the reader sensor is typicallymade using a TMR sensor structure, which includes two ferromagneticlayers that are separated by a dielectric layer called a tunnel barrier.One of the ferromagnetic layers is referred to as a reference layer (RL)wherein the magnetization direction is fixed by exchange coupling withan adjacent antiferromagnetic (AFM) pinning layer. The secondferromagnetic layer is a free layer (FL) wherein the magnetizationvector can rotate in response to external magnetic fields to be eitherparallel or anti-parallel to the magnetic moment in the RL depending onthe magnetic field direction from the recording media. Digital datasequence made of “0” or “1” is translated into different magnetizationdirections on the recording media which is recorded by a write sensor ineach recording head. As the FL rotates, the resistance measured bypassing a current from the FL to the RL will change. The change inresistance is measured and used to decode the magnetization pattern fromthe recording media and reproduce the information that was recordedearlier.

In FIG. 1A, one example of a TMR reader is shown having a sensorstructure 6 formed between a lower shield 4 and an upper shield 7. Thedown-track cross-sectional view depicts a front side of the sensorstructure at an ABS 30-30, and a backside 6 e adjoining a dielectric(gap) layer 5 b. In a so-called bottom spin valve configuration for thesensor structure, bottom portion 6 a comprises a RL, and may alsoinclude one or multiple seed layers and an AFM layer on the seed layer(not shown). There is a tunnel barrier 6 b between the RL and a FL 6 f.Upper portion 6 c is a capping layer. In some designs, a part of the RLmay be recessed from the ABS.

Referring to FIG. 1B, an ABS view of the TMR reader in FIG. 1A isillustrated and shows magnetization 3 m in adjacent JSs 3 provide alongitudinal biasing effect to stabilize FL magnetization 6 m in theabsence of an external magnetic field. One or both of permanent magneticmaterial and soft magnetic material each having a magnetization alignednear the FL are generally used to bias the FL magnetization moment withrespect to the RL so as to obtain a substantially orthogonal relativeorientation between FL magnetization 6 m and RL magnetization 6 n in azero applied field environment.

FIG. 1C depicts a top-down view of the sensor structure in FIG. 1B wherelayers above the FL are removed. A JS 3 is formed adjacent to each sideof FL 6 f at the ABS 30-30. The longitudinal biasing scheme provides aJS magnetization 3 m that is parallel to the ABS and to the FL magneticmoment 6 m so that a single domain magnetization state in the FL will bestable against all reasonable perturbations when no external magneticfield is applied. An inner JS side 3 s 1 is usually separated from theFL by a dielectric layer 5 a. Each JS also has a front side at the ABS,a backside 3 e, and an outer side 3 s 2 facing away from the FL. Notethat the cross-track direction along the y-axis is known as thelongitudinal direction, and a direction orthogonal to the ABS (along thex-axis) is referred to as the transverse direction.

Asymmetry of the quasi-static test (QST) response of a TMR read head isdefined as the relative difference in the reader resistance for positiveand negative magnetic fields (of equal magnitude) in the transversedirection. QST asymmetry, which is hereinafter referred to as asymmetry,is strongly dependent on the aspect ratio (AR) of the reader, which isdefined as the ratio of stripe height (SH) to FL width (FLW) expressedas SH/FLW where SH is the distance between a front side of the FL 6 f atABS 30-30 and the FL backside 6 e. Asymmetry increases as AR increasesthereby making the reader performance sensitive to process induced ARvariations, and establishing an upper limit to allowable AR set byacceptable asymmetry.

Asymmetry of reader response depends, among other factors, on therelative magnetization directions of the FL and the RL in the absence ofan applied magnetic field (zero field). Thus, FL magnetization directionin a zero field environment is affected by the strength of thelongitudinal bias, given by the magnitude of 3 m, and the FL AR. Thezero field relative magnetization directions of the FL and RL arequantified using the so-called “bias point”, which is quantified by theresistance of the sensor structure stack at zero field relative to thatat a very large external applied field when the FL and RL magnetizationsare driven parallel to each other. Because asymmetry becomesconsiderably large for long SH dimensions that lead to an AR of 1 orhigher, and contributes to degraded TMR reader performance, especiallywhen FLW is proximate to 30 nm or less, a method to improve theaforementioned sensitivity to increasing AR or SH is needed.

SUMMARY

One objective of the present disclosure is to provide a JS configurationto stabilize a FL magnetization in a sensor structure of a TMR readerthat compensates for higher asymmetry as AR increases above 1,especially for FLW proximate to 30 nm or less, without introducing asignificant loss in amplitude for low AR or SH readers.

A second objective of the present disclosure is to provide a method offorming the JS configuration that satisfies the first objective.

According to one embodiment of the present disclosure, these objectivesare realized with JS structure configured to have at least a first JSlayer having decreasing thickness with increasing height from the ABS.In some embodiments, two JS layers are coupled through antiferromagneticcoupling (AFC) on each side of a TMR sensor at an ABS. Thus, a first(lower) JS layer has a first magnetization (m1) aligned parallel to theABS and to a magnetization (m2) in the adjacent free layer, and providesthe primary means of longitudinally biasing (stabilizing) so that m2 isaligned orthogonal to the RL magnetization in the absence of anexternally applied magnetic field. There is a first AFC layer with asubstantially uniform thickness on the lower JS layer, and a second JSlayer on the first AFC layer where the second JS layer has amagnetization m3 aligned opposite to that of m1.

In a first embodiment, the lower JS layer has a tapered top surface suchthat a first thickness (t1) of the lower JS layer at the ABS is greaterthan a second thickness (t2) at a backside thereof at a first heightfrom the ABS. The second JS layer also extends to a backside at thefirst height from the ABS, and has a constant thickness between a frontside at the ABS and the backside. In other words, the second JS layertop surface is also tapered and formed essentially parallel to the lowerJS layer top surface. As the taper angle α between the lower JS layertop surface and a plane formed orthogonal to the ABS increases for agiven t2 and SH, the average thickness of the lower JS layer (t1+t2)/2increases (since t1≅t2+SH×tan α, assuming JS height is approximatelyequal to the SH of the FL), so the effective longitudinal bias on the FLfor a given SH of the FL increases. Also, for a higher value of α, for agiven t2, t1 increases at a faster rate with the SH of the FL (sincet1≅t2+SH×tan α), hence the rate of increase of effective longitudinalbias with AR or SH also becomes higher.

In a second embodiment, the second JS layer in the first embodiment ismodified to have increasing thickness with increasing height from theABS such that a backside thereof has a third thickness (t3) and a frontside at the ABS has a fourth thickness (t4) where t4<t3. Preferably,t1+t4=t2+t3 such that a top surface of the second JS layer is parallelto the lower JS layer bottom surface.

According to a third embodiment, the JS structure of the firstembodiment is modified to include a second AFC layer having a uniformthickness on the second JS layer, and a third JS layer with a uniformthickness on the second AFC layer. Therefore, both of the second andthird JS layers have tapered top surfaces that are essentially parallelto the top surface of the lower JS layer. Moreover, the third JS layerhas a magnetization m4 that is aligned parallel to m1 and m2, andopposite to m3.

In a fourth embodiment, the JS structure of the second embodiment ismodified to include a second AFC layer having a uniform thickness on thesecond JS layer, and a third JS layer with a uniform thickness (t5) onthe second AFC layer. As a result, a combined thickness (t1+t4+t5) ofthe JS layers at the ABS is essentially equal to a combined thickness(t2+t3+t5) of the JS layers at a backside of the JS structure such thata top surface of the third JS layer is parallel to the lower JS layerbottom surface.

Besides a FL, the sensor structure has a bottom portion comprised of abottommost seed layer, an AFM layer, and a synthetic antiparallel (SyAP)RL in some embodiments. There is a tunnel barrier between the RL and theFL, and a capping layer on the FL. The sensor structure has a bottomsurface formed on a bottom shield, a top surface that adjoins a topshield, and sidewalls connecting the top and bottom surfaces. Anon-magnetic insulation layer adjoins the sidewall of the sensorstructure and extends along a top surface of the bottom shield toseparate the JSs from the bottom shield and sensor structure.

In the JS structure, each of the magnetic layers may be made of a softmagnetic material such as Ni-rich NiFe, or Co-rich CoFe with amagnetization that is set during an annealing step with in situ magneticfield, or may be comprised of a hard magnetic material including one ofCoPt, CoCrPt, or FePt. Each of the antiferromagnetically coupled layersmay be separated using Ru, for example, with a thickness that inducesAFC between the JS layers.

In another embodiment that features a single JS layer, the taperedsingle JS layer compensates for a shift in bias point of the TMR readeras AR increases. The rate of compensation of asymmetry with increasingAR or SH depends on the taper angle of the single JS layer top surfacefor a given t2, and increases as taper angle becomes larger. As aresult, the reader design can accommodate AR values above 1 withoutintroducing unacceptably large asymmetry for large AR, or unacceptablylow amplitude for low AR.

The present disclosure also includes a method of forming a TMR sensorwith adjacent junction shields wherein at least the lower JS layer has atapered top surface. Once a sensor stack of layers with sidewalls andbackside is formed on the bottom shield, an insulation layer and firstJS layer are sequentially formed at each sidewall such that the first JSlayer has a top surface orthogonal to the eventual ABS plane. A taper isthen formed on the lower JS layer with an angled ion beam etch beforethe remainder of the JS structure is deposited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a down-track cross-sectional view of a sensor structure in aTMR reader according to one embodiment of the present disclosure.

FIG. 1B is an ABS view of the sensor structure in FIG. 1a that includesa JS on each side of the sensor in order to stabilize the FLmagnetization in a longitudinal (cross-track) direction.

FIG. 1C is a top-down view of the JSs and FL in FIG. 1B depicting theFLW and SH that determine AR.

FIG. 2 is a down-track cross-sectional view of a TMR reader having asensor structure formed at an ABS.

FIG. 3 is an ABS view of a sensor structure wherein FL magnetization isstabilized by two JSs that are antiferromagnetically coupled on eachside of the FL according to an embodiment of the present disclosure.

FIG. 4 is an enlargement of the bottom portion of the sensor structurein FIG. 3 that shows a synthetic antiparallel (SyAP) configuration forthe RL.

FIG. 5 is a down-track cross-sectional view of a pair of JSs in theprior art having a constant thickness with increasing height from theABS.

FIG. 6 is a down-track cross-sectional view of a pair ofantiferromagnetically coupled JSs wherein a lower JS is tapered to havedecreasing thickness with increasing height from the ABS while the upperJS has a constant thickness according to a first embodiment of thepresent disclosure.

FIG. 7 is a down-track cross-sectional view of the antiferromagneticallycoupled junction shields in FIG. 6 wherein the upper JS is modified tohave increasing thickness with increasing height from the ABS so that acombined thickness of the junction shields is constant (independent ofheight) according to a second embodiment of the present disclosure.

FIG. 8 is an ABS view of a sensor structure wherein FL magnetization isstabilized by three junction shields that are antiferromagneticallycoupled on each side of the FL according to an embodiment of the presentdisclosure.

FIG. 9 is a down-track cross-sectional view of the three junctionsshields in FIG. 8 wherein a lower JS is tapered to have decreasingthickness with increasing height from the ABS while middle and upperjunction shields have a constant thickness according to a thirdembodiment of the present disclosure.

FIG. 10 is a down-track cross-sectional view of theantiferromagnetically coupled junction shields in FIG. 9 wherein themiddle JS is modified to have increasing thickness with increasingheight from the ABS so that a combined thickness of the three junctionshields is constant (independent of height) according to a fourthembodiment of the present disclosure.

FIG. 11 is an ABS view of a sensor structure wherein FL magnetization islongitudinally stabilized by a single tapered JS formed on each side ofthe FL according to an embodiment of the present disclosure.

FIG. 12 is a down-track cross-sectional view of the single tapered JSshown in FIG. 11.

FIG. 13 is a plot of TMR reader transfer curves from micromagneticsimulations showing that a tapered JS according to the presentdisclosure compensates for increased asymmetry at higher AR unlike theresults observed with a uniform JS in the prior art.

FIG. 14 shows a plot of asymmetry vs. AR from micromagnetic simulationsfor a tapered JS, and a uniform JS in a TMR reader.

FIGS. 15-20 depict a series of process steps for fabricating a taperedJS adjacent to a sensor structure according to a process of the presentdisclosure.

DETAILED DESCRIPTION

The present disclosure is a JS design in a read head, and a method formaking the same, for longitudinal biasing of an adjacent FL in a TMRsensor wherein at least a first JS layer has a tapered top surface tocompensate for higher asymmetry as the AR increases above 1, especiallyas FLW shrinks to around 30 nm or less. Thus, an effectively thinner JSfor low AR ensures high amplitude if the reader has a low AR, while aneffectively thicker JS for high AR above 1 ensures that the asymmetry isnot unacceptably high. The JS design is flexible to include multiplemagnetic layers that are antiferromagnetically coupled. In the drawings,the x-axis (transverse) direction indicates a height dimension withrespect to the ABS, the y-axis is a cross-track (width) direction, andthe z-axis is a down-track (thickness) direction.

Referring to FIG. 2, one embodiment of a read head having a TMR sensorand JS structure of the present disclosure is depicted in a down-trackcross-sectional view from a plane orthogonal to an air bearing surface(ABS) 30-30. It should be understood that the read head may be part of acombined read head-write head design wherein the write head portion (notshown) is formed on the read head. The read head is formed on asubstrate 1 that may be comprised of AlTiC (alumina+TiC). Substrate 1 istypically part of a slider (not shown) formed in an array of sliders ona wafer. After the read head (or combined read head-write head) isfabricated, the wafer is sliced to form rows of sliders. Each row istypically lapped to afford an ABS before dicing to fabricate individualsliders that are used in a magnetic recording device.

The read head typically has a bottommost insulation layer 2 that isformed on substrate 1 and is made of a dielectric material such asalumina. A bottom shield 4 also referred to as the S1 shield is formedon insulation layer 2 and may be comprised of NiFe, CoFe, or CoFeNi. Insome embodiments, the top shield is a so-called S2A shield. Amagnetoresistive element also known as TMR sensor 6 is formed betweenthe top and bottom shields and usually has a front side exposed at theABS 30-30. There is a second insulation layer 5 b between the bottomshield and a top shield 7, and behind the TMR sensor. The read gap isdefined as the distance between the top shield and bottom shieldmeasured at the ABS.

Above the top shield 7, an insulation layer 8 and a shield (S2B) layer 9are sequentially formed. Shield layers 7, 9 may be made of the samemagnetic material or a different material as in the S1 shield 4, andinsulation layer 8 may be the same dielectric material or a differentmaterial as in insulation layer 2. The present disclosure anticipatesthat various configurations of a write head may be employed with theread head structure disclosed herein. Accordingly, write head layers arenot illustrated in order to focus on the key features of the junctionshields and TMR sensor embodiments disclosed with respect to FIGS. 3-12.

Referring to FIG. 3, an enlarged section of the read head in FIG. 2 isdepicted from an ABS perspective. The sensor structure 6 comprises abottom portion 6 a, tunnel barrier 6 b, FL 6 f, and a capping layer 6 cthat are sequentially formed on a top surface of bottom shield 4.Insulation layer 5 a contacts sensor sidewalls 6 s, and contacts the topsurface of bottom shield 4 that is not covered by the sensor structure.An optional seed layer 11 is formed on a top portion of insulation layer5 a above bottom shield 4. The exemplary embodiment depicts a bottomspin valve configuration wherein layer 6 a comprises a RL that is formedbelow the tunnel barrier in the sensor structure.

In FIG. 3, a first embodiment of the JS structure 12 of the presentdisclosure is shown and comprises a lower JS layer 13-1, first AFC layer15, and a second JS layer 14 sequentially formed on the optional seedlayer 11 on each side of the sensor structure. The seed layer may beselected from NiCr, NiFeCr, NiFe, Cu, or Ni, or bilayer or multilayerstacks comprising Ta, Ru, and one or more of the aforementioned seedlayer materials. Sidewall 12 s of the JS structure adjoins insulationlayer 5 a. Each lower JS layer has magnetization 13 m aligned parallelto FL magnetization 6 m thereby providing the primary means oflongitudinal biasing (stabilization) to FL 6 f in the absence of anexternally applied magnetic field. Each second JS layer hasmagnetization 14 m aligned opposite to 13 m as a result of AFC layer 15.AFC between layers 13-1, 14 has a net effect of stabilizing the JScompared with a single JS layer described later with regard to FIGS.11-12. The junction shields prevent stray magnetic fields produced by amagnetic medium (not shown) from inadvertently switching themagnetization direction of the FL.

Although bottom portion 6 a is depicted with a larger cross-track widththan that of capping layer 6 c, sidewalls 6 s in other embodiments maybe substantially vertical. Both shields 4 and 7 may be comprised of oneor more of NiFe, CoFe, or CoFeNi, or other magnetic alloys thereofcontaining additional elements. JS layers 13-1, 14 are made of a softmagnetic material such as one or more of CoFe, NiFe, and CoFeNi, orother magnetic alloys thereof containing additional elements. Inalternative embodiments, one or both JS layers are comprised of a hardmagnetic material including one or more of CoPt, CoCrPt, FePt, and thelike. AFC layer 15 is preferably Ru with an appropriate thickness.

According to one embodiment shown in FIG. 4, bottom portion 6 a in theTMR sensor structure may comprise a bottommost seed layer 20, AFM layer21, and a SyAP RL with an AP2/AFC layer/AP1 configuration. AP1 layer 24contacts a bottom surface of tunnel barrier 6 b and has a magnetization24 m out of the plane of the drawing. AP2 layer 22 isantiferromagnetically coupled to the AP1 layer 24 through AFC layer 23and has magnetization 22 m aligned anti-parallel to 24 m. Furthermore,magnetization 22 m is pinned in a fixed direction by AFM layer 21. Notethat AP1 magnetization 24 m is aligned orthogonal to FL magnetization 6m in the absence of an externally applied field (zero fieldenvironment). Each of the magnetic layers 6 f, 22, 24 may be comprisedof one or more of Co, Fe, and Ni, including alloys with B, Ta, Cr, or W.AFM layer 21 is generally comprised of PtMn, IrMn, or another suitableAFM material.

In related U.S. Pat. No. 9,230,577, we disclosed a JS structure similarto that shown in FIG. 3 where multiple JS layers areantiferromagnetically coupled in order to improve JS stability. However,the earlier JS structure did not consider the effect of an AR higherthan 1 that causes unacceptably high asymmetry in the read head design,especially as advanced designs are based on increasingly smaller FLWvalues proximate to 30 nm, or less. Now, we have discovered a JSstructure capable of compensating for higher asymmetry in advancedproducts with small FLW, without reducing the amplitude to an extentthat is unacceptable for low AR designs. The improved structure isprimarily evident in down-track cross-sectional views along a plane thatis aligned orthogonal to the ABS.

FIG. 5 illustrates a JS structure having two magnetic layers 13, 14 thatare antiferromagnetically coupled through an intermediate AFC layer asfound in one of our process of record (POR) schemes. Both JS layers 13,14 have a constant thickness t2′, t3′, respectively, between front sides13 f, 14 f at the ABS 30-30, and backsides 13 e, 14 e, respectively.Lower JS layer 13 has top surface 13 s aligned orthogonal to the ABS.When the AR becomes greater than 1 (SH>FLW in FIG. 1c ), shapeanisotropy of the FL increases in the transverse direction and leads toan effective decrease in longitudinal bias (LB). As a consequence, thereis a shift in the zero-field magnetization direction of the FL and ashift in the bias point as the FL magnetization rotates closer to the RLmagnetization direction thereby making the zero-field angle between theFL and RL to be less than 90°. Departure from an orthogonal alignment ofFL and RL magnetization directions in a zero field environment causesnon-linear response of the reader to applied magnetic fields, and anincrease in asymmetry for increasing AR values.

With regard to FIG. 6 that is a cross-sectional view taken along plane50-50 in FIG. 3, we have discovered that by incorporating a non-uniformthickness in the transverse direction for the lower JS layer, there isan effective increase in LB on the FL with increasing AR. In otherwords, the effective LB increase in embodiments of the presentdisclosure compensates, at least partially, for the effective decreasein LB in sensor designs when AR reaches 1 or higher, without degradingthe amplitude for low AR readers (AR<1) with thicker than necessary JS.The lower JS layer 13-1 is designed to have sloped top surface 13 t suchthat thickness t1 at a front side 13 f at the ABS 30-30 is greater thanthickness t2 at the backside 13 e. Thickness t2 may be different fromt2′ in the POR scheme in FIG. 5. Thus, the effective or averagethickness (t1+t2)/2 in the FIG. 6 scheme may be smaller or greater thant2′ for a conventional reader in FIG. 5, depending on how t1 and t2compare with respect to t2′. As explained later, t2 is preferablysubstantially larger than thickness t3 of the second JS layer 14 toensure a higher magnetization 13 m in lower JS layer 13-1 thanmagnetization 14 m in second JS layer 14. AFC layer 15 and JS layers13-1, 14 extend to the same height h from the ABS.

Top surface 13 t of lower JS layer 13-1 is formed at a taper angle αthat is greater than 0° with respect to plane 51-51 which is orthogonalto the ABS 30-30, and parallel to bottom surface 13 b. A maximum valuefor angle α is related to the practical aspects of an angled ion beametch (IBE) that forms the taper angle. For example, depending on thedesired values of t1 and h, the taper angle may be restricted to a valuesubstantially less than 45° in order to maintain t2>0. Preferably, AFClayer 15 with top surface 15 t is deposited in a conformal manner on thelower JS layer and has a uniform thickness. Moreover, second JS layer 14has a constant thickness t3 between a front side 14 f and backside 14 ethereof. Accordingly, top surface 14 t is also tapered and isessentially parallel to tapered top surface 13 t.

Compensation of shape anisotropy by incorporating an effectivelyincreasing LB with increasing AR for the tapered JS design of thepresent disclosure leads to a slower rate of bias point shifting withincreasing AR, and hence a slower rate of increase in asymmetry with AR.The rate of increase in longitudinal bias with increasing AR or SH isdetermined by the taper angle α. In particular, the rate of increase ofLB with AR increases as the taper angle α becomes larger, for a givent2. Note that the slope of the reader transfer curve shown later in FIG.13, which decides the so-called amplitude of reader response, may betuned by the average thickness of the tapered lower JS layer 13-1,namely, (t1+t2)/2, for a given amount of taper (α) and a given thicknesst3 of the upper JS layer 14. Thus, for a certain taper angle α and SH ofthe FL, the effective LB becomes larger as the average thickness(t1+t2)/2 of the tapered lower JS layer increases.

Referring to FIG. 7, a second embodiment of the present disclosure isillustrated and retains all of the features of the first embodimentexcept a second JS layer 14-1 replaces JS layer 14 and has increasingthickness with increasing height from the ABS 30-30. The second JS layernow has a thickness t4 at a front side 14 f at the ABS, and thickness t3at the backside 14 e where t4<t3, and t222 t3. Preferably,(t1+t4)=(t2+t3) such that a top surface 14 t of the second JS layer isorthogonal to the ABS and parallel to lower JS layer bottom surface 13b. The effect of larger effective longitudinal bias on slowing theincrease in asymmetry with ARs above 1 is maintained from the firstembodiment. Moreover, for a given taper angle α, the rate of increase ofLB with increasing AR or SH is greater in the FIG. 7 embodiment than inthe first embodiment (FIG. 6) since the net LB is given by thedifference in moment of the lower JS and the upper JS layer.

The strength of the LB on the FL is primarily determined by the neteffective moment or difference in magnitudes of the moments (13 m-14 m)of the JS layers in the longitudinal direction. When lower JS layer 13-1is a material with saturation magnetization Ms₁ and JS layer 14-1 is amaterial with saturation magnetization Ms₂, magnitude of magnetization13 m is proportional to Ms₁×(t1+t2)/2 and is purposely made higher thanthe magnitude of the magnetization 14 m, which is proportional toMs₂×(t3+t4)/2 to give a net longitudinal bias parallel to that of the FLmagnetization (6 m in FIG. 3).

In the second embodiment, in addition to the lower JS layer 13-1becoming effectively thicker with increasing AR, the upper JS layer 14-1is effectively thinner with increasing AR. Thus, the effective magnitude(saturation magnetization× thickness) of 14 m subtracted from that of 13m increases at a faster rate with increasing AR compared with thesituation in FIG. 6 where the effective magnitude (saturationmagnetization× thickness) of 13 m increases with AR, but that of 14 m isconstant with increasing AR. In other words, adjusting the lower JSlayer to have thickness t1 at the ABS where t1>t2, and adjusting thethinner second JS layer to have thickness t4 at the ABS where t4<t3,ensures a larger rate of increase in LB with increasing AR than the JSstructure in FIG. 6 of the first embodiment, and hence offers a largerrate of compensation of asymmetry with increasing AR or SH.

In all of the aforementioned embodiments relating to JS 12, all JSlayers extend to the same height h from the ABS where h is preferablygreater than or equal to the SH (not shown) of the FL to ensure amaximum LB along the entire height (SH) of each side 6 s of free layer 6f.

According to another embodiment illustrated in the ABS view in FIG. 8,the JS structure 12 in the previous embodiments may be replaced by JSstructure 18 with sidewall 18 s where a second AFC layer 16 is formed onsecond JS layer 14, and a third JS layer 17 is disposed on the secondAFC layer. All other aspects of the sensor structure relating to layers6 a, 6 b, 6 f, and 6 c in FIGS. 3-4 are retained. The third JS layer hasa magnetization 17 m that is aligned parallel to magnetization 13 m inthe lower JS layer 13-1, and is advantageously used to further stabilizethe junction shield structure, especially when magnetization 17 m ispinned by ferromagnetic coupling with magnetization 7 m in the topshield. In summary, magnetization 17 m is antiferromagnetically coupledto magnetization 14 m that in turn is antiferromagnetically coupled tomagnetization 13 m to stabilize JS structure 18. Here, magnetization 13m continues to provide the primary contribution for longitudinal biasingof FL magnetization 6 m, and is designed to have a greater (Ms×thickness) value than that of JS layers 14, 17. Down-trackcross-sectional views relating to the structure in FIG. 8 are describedwith respect to FIGS. 9-10.

In FIG. 9, a third embodiment of the JS structure of the presentdisclosure is depicted from a down-track cross-sectional view at plane50-50 in FIG. 8. In particular, the lower JS layer 13-1 and second JSlayer 14 features are maintained from the first embodiment where taperedtop surface 13 t is formed at an angle α with respect to plane 51-51,and AFC layer 15 with a uniform thickness and top surface 15 t, andsecond JS layer 14 with constant thickness t3 are sequentially formedthereon. In addition, the second AFC layer 16 with top surface 16 tpreferably has a uniform thickness on a top surface 14 t of the secondJS layer. Moreover, the third JS layer 17 with a front side 17 f at theABS 30-30, and a backside 17 e at height h has a constant thickness t5with increasing height from the ABS. Preferably, both t3 and t5 are lessthan t2. Accordingly, top surface 17 t is tapered and is essentiallyparallel to top surfaces 13 t and 14 t of JS layers 13-1, 14,respectively. In the exemplary embodiment, all layers in JS structure 18extend to the same height h from the ABS where h is preferably greaterthan or equal to the FL SH (not shown) to ensure a maximum LB along theentire height (SH) of each side 6 s of FL 6 f.

Referring to FIG. 10, a fourth embodiment of the JS structure of thepresent disclosure is depicted from a down-track cross-sectional view.Here, the lower JS layer 13-1 and second JS layer 14-1 features aremaintained from the second embodiment where tapered top surface 13 t isformed at an angle α with respect to plane 51-51. As describedpreviously, AFC layer 15 with top surface 15 t has a uniform thickness,and second JS layer 14-1 has a thickness t4 at the ABS that is less thanthickness t3 at a backside 14 e thereof. The JS structure in the secondembodiment is modified with the addition of a second AFC layer 16 havinga uniform thickness on a top surface 14 t of the second JS layer, andforming the third JS layer 17 with a front side 17 f at the ABS 30-30,and a backside 17 e with a constant thickness t5 on the second AFClayer. As a result, a combined thickness (t1+t4+t5) of the JS layers atthe ABS is preferably equal to the combined thickness (t2+t3+t5) of theJS layers at the JS backside, and top surface 17 t is essentiallyparallel to lower JS layer bottom surface 13 b. The benefit of slowingan increase in asymmetry with increasing AR above 1 is also realized inaforementioned third and fourth embodiments.

The present disclosure also encompasses an embodiment depicted in FIG.11 where JS 12 on each side of the sensor structure has a single JSlayer 13-1 with magnetization 13 m aligned parallel to magnetization 6 min FL 6 f. Otherwise, all aspects of the read head structure in earlierembodiments are retained.

As indicated by the down-track cross-sectional view in FIG. 12, topsurface 13 t of JS layer 13-1 is tapered and has a thickness t1 at afront side 13 f at the ABS 30-30 that is greater than thickness t2 atbackside 13 e as described previously with regard to the first fourembodiments. Although the single JS layer embodiment may be less stablethan the JS structure in earlier embodiments where at least two JS layerare antiferromagnetically coupled, the tapered JS layer is expected toprovide an advantage in limiting the rate of increase in readerasymmetry as AR (SH/FLW) increases above 1.

To confirm the advantage of incorporating a JS structure having atapered top surface on a lower JS layer according to the firstembodiment depicted in FIG. 6, full micromagnetic simulations wereperformed and compared with results from a POR design in FIG. 5 wherethicknesses for both the lower JS layer and second JS layer are constantwith increasing height from the ABS. The geometry of both of the FIG. 6design and POR design has been defined and meshed, and the mesh in bothcases was exported using commercial finite element modeling software.The Landau-Lifshitz-Gilbert equation (1) below that predicts theprecessional motion of magnetization M in a solid including the rotationof magnetization in response to torques has been solved in the finiteelement mesh using commercial micro-magnetic simulation software.d{right arrow over (M)}/dt=−|γ|{right arrow over (M)}×{right arrow over(H)} _(eff) −|γ|α/M _(s) {right arrow over (M)}×({right arrow over(M)}×{right arrow over (H)} _(eff))   (1)

In equation (1), {right arrow over (M)} is the magnetization vector,M_(s) is the saturation magnetization, {right arrow over (H)}_(eff) isthe effective field, γ is the Landau-Lifshitz gyromagnetic ratio, α isthe damping constant, and t is time.

For the simulation, the top surface 13 t of the lower JS layer 13-1 isassumed to have a 3° angle α with respect to plane 51-51 so that thelower JS layer is approximately 1.5 nm thicker at the ABS 30-30 than atbackside 13 e where t2 is 10 nm in this example (with effective heightof JS being proximate to FL SH and for FL AR˜1). The thickness of secondJS layer 14 was set at 1.5 nm and FLW=30 nm. Inputted Ms values forvarious layers are the following: 1.07 Tesla (T) for JS layer 13-1; 1.77T for JS layer 14; 1.64 T for the AP1 RL; and 0.935 T for the FL. Asmentioned earlier, increasing the taper angle α above 3° will provide alarger rate of compensation of asymmetry as AR increases. Details of thegeometry and material parameters of the remainder of the pinning stackand the shields in the model are not critical with respect to the mainfocus of the present disclosure, and are omitted for the sake of brevityand simplicity.

FIG. 13 shows the simulated transfer curves of the sensor structure inthe read head for an adjacent JS structure of the first embodiment, andfor the POR JS design in FIG. 5. The negative value of the scalarproduct of the FL and AP1 RL magnetizations shown as m (−m_(AP1)·m_(FL))where m_(AP1) and m_(FL) are magnetizations 24 m and 6 m, respectively,in FIGS. 3-4, is proportional to the resistance of the sensor, and isplotted as a function of the externally applied uniform field in thetransverse direction (H_(X)). We observe that the bias point (determinedby the dot product m_(AP1)·m_(FL) at zero applied field or H_(X)=0)shifts at a slower rate to the parallel state (−m_(AP1)·m_(FL)=−1) as ARincreases from 0.8 to 1.0, 1.2, and 1.4, respectively, for the taperedJS structure compared with the POR design. Note that curve 40 (taperedJS at AR=0.8), curve 42 (tapered JS at AR=1.0), curve 44 (tapered JS atAR=1.2), and curve 46 (tapered JS at AR=1.4) each have a less negativeproduct (−m_(AP1)·m_(FL)) than curve 41 (POR at AR=0.8), curve 43 (PORat AR=1.0), curve 45 (POR at AR=1.2), and curve 47 (POR at AR=1.4),respectively, at H_(X)=0.

In FIG. 14, asymmetry of the QST response (relative difference betweenreader resistance at H_(X)=600 Oe and H_(X)=−600 Oe) is plotted as afunction of AR for the POR JS design in FIG. 5 (curve 61), and thetapered JS of the first embodiment in FIG. 6 (curve 60). Clearly, thetapered JS design where angle α=3° has a smaller rate of increase inasymmetry for ARs greater than 1 compared with the conventional JSstructure.

The present disclosure also encompasses a method of forming a sensorwith adjacent JS structures comprised of a tapered lower JS layer. InFIG. 15, a first step in a fabrication process is depicted wherein abottom shield 4 is formed on a substrate (not shown) by a platingmethod, for example. Thereafter, layers 6 a, 6 b, 6 f, and 6 c in thesensor stack are sequentially formed on a top surface of the bottomshield by a sputter deposition process. According to one embodiment thatrepresents a bottom spin valve configuration illustrated in FIG. 4,layer 6 a is comprised of a lower seed layer, a middle AFM layer, and anuppermost SyAP RL with an AP2/AFC layer/AP1 configuration on the AFMlayer. Note that in some designs, one or both of the AFM layer and theSyAP RL may be recessed from the ABS entirely or in part, requiringadditional process steps to be defined.

During the following step in the fabrication sequence, a photoresistlayer is spin coated and patterned on the sensor top surface 6 t by aphotolithography process to generate a pattern including a photoresistisland 58 having a width w in the cross-track direction. The photoresistpattern typically includes a plurality of islands arranged in rows andcolumns from a top-down view that is not shown in order to focus on thekey features in the drawing. There are openings 57 a, 57 b on eitherside of the photoresist island that expose substantial portions of topsurface 6 t. A large portion of top surface 6 t is also uncovered alonga backside (not shown) of island 58 to completely isolate adjacentislands in the photoresist pattern.

According to one embodiment in FIG. 16, each island 58 may have asubstantially rectangular shape from a top-down view with a stripeheight SH along sidewalls 58 s. A front side 58 f of each island isformed along plane 30-30 that indicates the eventual position of the ABSafter a lapping process is performed as appreciated by those skilled inthe art. In an alternative fabrication flow, two separatephotolithography steps may be employed to pattern the cross-trackdimension w, and the SH dimension. Moreover, the patterned photoresistisland 58 may extend to an opposite side of plane 30-30 with respect tobackside 58 e since the SH dimension may be further defined by thesubsequently formed FL during the aforementioned lapping process. Insome embodiments where sidewalls 6 s in FIG. 3 (or FIG. 8) are proximateto 90° with respect to the top surface of bottom shield 4, cross-trackwidth w between sides 58 s is substantially equal to the desired FLW infree layer 6 f following a subsequent etch process.

Referring to FIG. 17, a reactive ion etch (RIE) or ion beam etch (IBE)process is performed to transfer the shape of the photoresist island 58bounded by openings 59 a, 59 b through the sensor stack of layers. Theetch process stops on a top surface 4 t of the bottom shield. Asmentioned previously, the exemplary embodiment shows non-verticalsidewalls 6 s. However, depending on the etch process and the desiredFLW, substantially vertical sidewalls 6 s may be formed in otherembodiments such that w is not significantly less than FLW. Also, asmentioned earlier, depending on the exact design of the reader, aportion of stack 6 a including the RL may be recessed from the ABS(plane 30-30 in FIG. 16).

Referring to FIG. 18, insulation layer 5 a is deposited along thesidewalls 6 s and on exposed portions of top surface 4 t, including aportion 5 b behind the patterned TMR sensor stack. In alternativeprocess flows, the portion of the insulation layer 5 b at a backside ofthe FL (not shown) below photoresist island 58, and portions of theinsulation layer 5 a along the sides of the FL may be formed usingseparate photolithography and deposition processes. Next, seed layer 11and lower JS layer 13-1 having sidewalls 13 w are sequentially depositedon the insulation layer with an ion beam deposition (IBD) method or thelike. A second photoresist layer is coated, patternwise exposed, anddeveloped to yield a rectangular shaped island 59 having a cross-trackwidth c between sides 59 s. Island 59 has a front side 59 f that is onan opposite side of plane 30-30 with respect to island 58, and isrecessed a distance b from plane 30-30. In embodiments where photoresistisland 58 extends beyond the eventual ABS plane as mentioned previouslywith regard to FIG. 16, some or all of photoresist island 59 may beformed on a top surface of photoresist island 58.

Referring to FIG. 19, a down-track cross-sectional view of the layout inFIG. 18 is shown along plane 55-55 and shows an exposed top surface 13t. A key feature is an angled IBE where ions 36 are directed at topsurface 13 t at a taper angle α. The ions may be generated from a noblegas that is Ar, Kr, Ne, or Xe.

FIG. 20 shows the result of the IBE in FIG. 19, and subsequent removalof photoresist islands 58, 59 by a conventional stripping process.Thereafter, AFC layer 15, and JS layer 14 are conformally deposited ontop surface 13 t to provide the JS structure depicted in the firstembodiment shown in FIG. 6.

With regard to the second embodiment depicted in FIG. 7, the AFC layer15 is deposited on the tapered top surface 13 t of lower JS layer 13-1.Then, upper JS layer 14-1 is deposited on the AFC layer. A chemicalmechanical polish (CMP) step or an alternative planarization method maybe performed to yield an upper JS layer top surface 14 t that iscoplanar with top surface 6 t of the TMR sensor.

Thereafter, a top shield is formed with a conventional process tocomplete the shield structure depicted in FIG. 3. As mentionedpreviously, a second anneal step may be performed to set the directionof JS layer 13-1. However, the anneal conditions should not be toostrenuous in order to avoid altering the magnetization direction 6 m infree layer 6 f in the sensor structure. After all layers in the readhead or combined read/write head are formed, a conventional lappingprocess is employed to form the ABS at plane 30-30.

In summary, all of the embodiments described herein may be accomplishedwith materials and processes used in the art. Therefore, enhanced TMRsensor performance for ARs above 1 is realized by compensating thelongitudinal bias to slow the increase in asymmetry compared with priorart JS schemes. The benefits of the present disclosure are achieved withno considerable additional cost compared with current fabricationschemes that produce JS structures wherein each magnetic layer has auniform thickness.

While this disclosure has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this disclosure.

We claim:
 1. A read head, comprising: (a) a tunneling magnetoresistive (TMR) sensor structure that has a bottom surface formed on a bottom shield, a top surface contacting a top shield, and two sidewalls connecting the top and bottom surfaces, and comprising a free layer (FL) having a FL width (FLW) and a magnetization in a cross-track dimension between the sidewalls, and a stripe height (SH) between a FL front side at an air bearing surface (ABS) and a FL backside; and (b) a junction shield (JS) structure formed adjacent to each of the two sidewalls, and contacting the top shield, and that provides a longitudinal bias to the FL magnetization, and with a single JS layer having a bottom surface aligned orthogonal to the ABS, and a tapered top surface that is planar and extends from the ABS to a backside of the JS layer, and formed at a taper angle greater than 0 degrees with respect to a first plane that is aligned orthogonal to the ABS and parallel to the single JS layer bottom surface, wherein a thickness of the single JS layer decreases with increasing height from the ABS such that there is a first thickness (t1) at the ABS that is greater than a second thickness (t2) at a backside of the single JS layer, and a first magnetization therein is parallel to the FL magnetization.
 2. The read head of claim 1 further comprised of an insulation layer formed between the single JS layer and the bottom shield, and between the JS structure and the sensor structure sidewalls.
 3. The read head of claim 1 wherein the single JS layer is comprised of CoFe, NiFe, CoPt, CoCrPt, or FePt.
 4. The read head of claim 1 wherein the single JS layer extends from the ABS to a first height that is greater than or equal to the stripe height (SH).
 5. The read head of claim 1 wherein the longitudinal bias applied to the free layer magnetization increases as the taper angle increases, for a given t2 and SH.
 6. The read head of claim 1 wherein increasing the taper angle provides a larger effective longitudinal bias for aspect ratios (ARs) having a (SH/FLW) greater than 1 thereby slowing a rate of increase in asymmetry compared with a JS structure having a uniform thickness in a single JS layer.
 7. The read head of claim 1 wherein the TMR sensor structure further comprises a synthetic anti-parallel reference layer having an AP2/AFC layer/AP1 configuration wherein the AP2 layer has a magnetization that is pinned by an adjoining antiferromagnetic layer, the AP1 layer adjoins an opposite side of a tunnel barrier layer with respect to the FL, and has a magnetization that is aligned anti-parallel to that of the AP2 layer, and orthogonal to the FL magnetization in an absence of an externally applied magnetic field. 